To be perfectly honest, men's necklaces are not a token of the dapper man's repertoire  but if wielded appropriately can greatly accent the sparkling eye of your lady friend, or provide a suitable means for displaying a personalized garment.

Get wounded in Nam? Receive the Medal of Honor?  If so you'd be a great candidate for a necklace.  The places in a dapper man's closet are few and far between but are there none-the-less.  If you choose to wear a necklace on daily basis, try to make it something that is meaningful and accurately represents your style.  It's obvious to me that most men who wear necklaces do so because they are trying to be part of some subculture.  Be your own subculture.